pondpaul pondpaul
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  • Posted on: 25/8/2013 14:23
Water Under Liner #1
Hello

With the heavy rain in Essex yesterday water from the fence behind my pond got a lot of water coming through and it found its way under the liner which rose and overflowed the pond. i think I just lost 1 fish before we spotted it.
what do people recommend I should do as it is still looks to have a lot of water under the liner.
My pond is about 9 feet by 5 feet and 26 inches deep and has about 80 fish.
thanks for any assistance.
regards Paul
finnipper finnipper
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  • Posted on: 25/8/2013 16:37
Re: Water Under Liner #2
This isn't easy to deal with at all, the best option would be to dig a hole next to the pond which if near enough to the pond will fill with water and this will allow it to be pumped away, but if the pond is very deep then this isn't really practical in which case you'll just have to allow the water to drain away naturally.
If it is a persistent problem you could build the pond sides up above ground level and the increased depth will make a higher pressure on the liner at the bottom and prevent water from seeping under the liner.
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  • Posted on: 25/8/2013 18:32
Re: Water Under Liner #3
pondpaul, what finnipper says is right in some respects, not in others. (Sorry finnipper)

The problem you have is that the local water table is too high.

Its not just because the water came from under the fence, water from everywhere sinks into the ground (where it can) and in so doing raises the water table.

The only real solution is do dig a hole about 15cm lower than the pond is deep, put a paving slab at the bottom of the hole, put a wide drainage pipe onto the slab, make it so that it is about 4cm below ground surface level.
Backfill around the pipe with gravel. Put a sump pump with float switch in the pipe. (So it stands on the paving slab) Run a pipe from the sump pump to waste.
Switch the pump on and watch, make sure the float switch can move easily, put another paving slab on top of the pipe. Done.

If you just dig a hole and put in a sump pump as the water is pumped out so the walls will collapse and the soil will be pumped away making the hole bigger, and bigger. the pipe and paving slabs prevent this.

Raising the level of the pond sounds to be a good idea, the down side is it can not be guaranteed to work, not to mention you will have to buy a new liner.

The reason is hydraulics. If the water table around your pond is rising and you build a wall around the liner you are in effect building a restriction, so by the laws of hydraulics you are making things worse.

To be honest, all you can realistically do now is prepare for the next time.
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herbie76 herbie76
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  • Posted on: 25/8/2013 20:36
Re: Water Under Liner #4
Same happened to my new pond this morning, biblical rain yesterday and reckon the water tables so high. The pond 5ft by 6ft and about 2ft deep was empty and almost turned inside out, water under the liner and all sorts! Pumped the water out into another pre formed pond, dug out another ledge and forked the ground, no water left so thought job done and leave liner to dry etc.

Gone out for a few hours no rain, come back got at least 3 inches of water just sat in bottom of pool where liner should be ! How do I resolve ? Do I dig simply dig down furthet in the original pond and the put a slab in it or dig an additional hole ?
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  • Posted on: 25/8/2013 20:42
Re: Water Under Liner #5
Quote:

herbie76 wrote:
..........How do I resolve ? Do I dig simply dig down furthet in the original pond and the put a slab in it or dig an additional hole ?


Read the post above yours, written by me.
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finnipper finnipper
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  • Posted on: 25/8/2013 20:49
Re: Water Under Liner #6
A couple of points to make my post clearer.

The first idea will only work if this is a one off problem due to the massive rainfall, if it is a persistent problem (as mentioned in the post) i.e. due to a high water table then it wouldn't work but the second idea would.
I'm not sure which laws of hydraulics you are referring to but
the extra depth creates extra pressure and the pressure would keep any water from coming between ground and liner. Yes it would require a new liner but it would not create any new problems.
The raised section doesn't have to be a wall, it could be something as simple as railway sleepers above ground.

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pondpaul pondpaul
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  • Posted on: 25/8/2013 21:26
Re: Water Under Liner #7
Thanks for the advice

I think it was the amount of rain coming down and there were many flooded places in this area so I think it water got somehow got under the liner from the top as I have never had this problem for many years'
we diverted the water away from the pond but the water was going across our grass about 2-3 inches deep running to our side passage and down to the drain in the road for many hours last night.

I will leave it a few days to settle down and then check what is under the liner and hope it drains away.

Paul
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  • Posted on: 16/9/2013 5:29
Re: Water Under Liner #8
Let me guess...you have clay soil?

finnipper has it.

I would only add that it isn't normally a high water table, but the exact same effect. A high water table can do it, but it has to be higher than the pond water level which is unusual. Normally when there's a high water table you run into that problem when digging the hole, unless the table happens to be low at the time.

More common, especially when I hear rain, is clay soil not draining.

Water is slow to drain thru clay soil so it only takes a little bit of water to get under a liner to fill all the space between liner and clay. When the water level in the pond is lower than the ground the entire pond and liner is like a boat in a lake and starts to float. Allows more water under, more floating, pretty soon the pond over flows putting more water under the liner.

A liner in a clay hole really compresses the clay. If you were to empty the pond and remove the liner and fill the hole with water it would probably whole water for a long time. It would look exactly like a high water table, but it would just be slow drainage.

Building up the sides like finnipper suggested is the easiest fix imo. The water level inside the pond just has to be even 1" above surrounding ground. Water still gets behind the liner, can't be helped, but can't get above ground level. So the water in the pond will always be heavier than the water under the liner can push up.